Various methods are known for measuring or evaluating the measurement output of foil strain gage transducers arranged in a bridge circuit. However, difficulties arise in such measuring methods because the strain gage transducer bridge circuit typically outputs a measurement voltage of only a few millivolts, even under maximum load, whereby this small measurement voltage must be amplified in corresponding measuring amplifiers for further processing, evaluation and transmission. In order that these analog measurement values can be transmitted and evaluated in microprocessor circuits with the lowest possible distortion or interference, the analog output signals are usually converted into digital measurement values, which are then provided to a microprocessor for further processing.
Such a typical measuring method for use in connection with a carrier frequency measuring amplifier, with transducers arranged in a bridge circuit, is described in M. Kochsiek, "Handbuch des Waegens" (Hand Book of Weighing), 2nd Edition, Braunschweig, Germany, 1989, page 551. The circuit and method according to this publication include an oscillator that generates an alternating voltage having a carrier frequency, which in turn is modulated with a measurement value by the transducer circuit. The resulting amplitude modulated carrier is then amplified in an input amplifier. This input amplifier is selective and transmits only frequencies in the range of the carrier frequency. The amplified carrier voltage is then demodulated, the remaining signal components of the carrier are filtered out, and the resulting measurement signal is finally provided to an output amplifier, which outputs the amplified measurement signal.
In order to allow further measured value transmission and processing in a microprocessor system, the above described analog output signal would have to be converted to a corresponding digital signal in an analog-to-digital converter. The demodulation carried out before the analog-to-digital conversion is typically carried out by means of a phase controlled rectifier, which always necessarily involves a loss or degradation of information, which in turn makes it difficult to achieve an exact calibration and evaluation of the measurement results.
German Patent Laying-Open Publication 4,417,228 (Altwein) also describes a carrier frequency measuring method with measuring transducers in a bridge circuit. According to this reference, before transmission of the measured value, the measurement signal is modulated by means of a modulation amplifier, amplified, and then again demodulated by means of a phase controlled bridge rectifier. However, in this process it must first be ensured before carrying out the demodulation, that the carrier frequency applied to the bridge is exactly in phase with the measurement voltage. For this reason, any existing phase differences are compensated or tuned-out by means of a manual phase compensation, upon beginning the signal processing. Since in practice new phase shifts arise over time, and such phase shifts lead to measurement inaccuracies, it is necessary to repeat the above-mentioned manual phase compensation measures at certain specified time intervals.